Ceramic arc tube and closure member construction



I Jan; 14, 1969 w. J. KNOCHEL ET AL 3,422,300

CERAMIC ARC TUBE AND CLOSURE MEMBER CONSTRUCTION Filed June 30. 1966 FIG.3. y ISZ 1Q 30 20 fi-"lmllh- VI 2 P 46 I6 541 22 LXI I I Lg-mumh- INVENTORS William J. Knochel,Froncis QM. Lin and Hugh D. Fraser ATTORNEY United States Patent 3,422,300 CERAMIC ARC TUBE AND CLOSURE MEMBER CONSTRUCTION William J. Knochel, West Orange, N.J., Francis C. M.

Lin, New York, N.Y., and Hugh D. Fraser, West Caldwell, N.J., assignors to Westinghouse Electric Corporation, Pittsburgh, Pa., a corporation of Pennsylvania Filed June 30, 1966, Ser. No. 562,017 US. Cl. 313-220 8 Claims Int. Cl. H01 17/16; H011 1/53; H01j 61/30 ABSTRACT OF THE DISCLOSURE A ceramic discharge device including a tubular elongated light transmissive ceramic envelope closed at each end by an end closure member carrying an internally directed electrode and having spaced between each electrode and its adjacent closure member an arc interceptor comprising either a plurality of radially extending spoke-like struts or a plurality of radially extending curved fins to prevent an arc from striking past the opposite electrode to the opposite end closure member during starting of the lamp. The exhaust-and-fill tubulation of the lamp may also be filled with a metallic plug to reduce the open area and thereby reduce the internal volume behind the electrode.

This invention relates to high temperature gas discharge lamps and more particularly to gas discharge lamps employing high density polycrystalline alumina envelopes and operating at intermediate pressures.

A recent innovation in metal vapor lamps has been the introduction of sintered, translucent, high-density polycrystalline alumina in the form of a tubular elongated envelope. To form a lamp, the tubular elongated body member or envelope is closed 01f at each end by electrodecarrying ceramic or metal discs or caps which are sealed directly to the envelope and confine a discharge-sustaining filling. The use of a translucent polycrystalline alumina body member permits operation at heretoforce inaccessible temperatures thus permitting a greater degree of ionization of the metal vapor medium. Such a lamp is disclosed in US. Patent No. 2,971,110 issued Feb. 7, 1961, to Kurt Schmidt for Metal Vapor Lamps.

When a ceramic end disc or cap is employed as a closure member closing off the ends of the polycrystalline alumina tube, some difficulty has been encountered during the starting of the lamp, due to the striking back of the arc from the electrode to the end disc or cap prior to the stable establishment of the are between the spaced electrodes. Striking back, refers to the striking of an are from one electrode to the larger area metallic or sodium coated ceramic end disc behind the opposite electrode prior to a suflicient rise in temperature to establish the arc between the opposed electrodes. This striking back of the arc between the electrode and an end disc has been known to cause damage to the ceramic end disc or cap due to the concentration of high temperature at the point of arc contact. When refractory metal end discs or caps are employed, the striking back from the electrode to the end cap is equally damaging due to the spot melting of the end disc or cap at the point of arc contact. It has also been found that, in some instances, where either ceramic or refractory metal end discs or caps are employed, the striking back occurs at the area of the seal between the end cap or disc and the sintered aluminum oxide tube thus causing cracking and failure at the point of seal.

It is accordingly an object of the present invention to prevent the striking back of an are between the electrode and end cap during the starting of metal vapor discharge lamps.

3,422,300. Patented Jan. 14, 1969 Another object of the present invention is to eliminate the destructive effect of an are being established between the electrodes and end caps of a high temperature metal vapor lamp prior to the establishment of an arc between the spaced electrodes.

These and other objects are accomplished in an electric discharge lamp employing a tubular elongated light transmissive ceramic envelope and end caps carrying a pair of spaced electrodes by providing between each electrode and its associated end cap a metallic means within the envelope to receive any are prior to the buildup of sufiicient pressure to permit the arc to transfer to the electrodes.

The foregoing, as well as other objects, and many of the attendant advantages of the present invention will be better understood as the following detailed description is considered in connection with the accompanying drawing in which;

FIGURE 1 is a sectional view of a polycrystalline alumina metal vapor lamp employing one embodiment of the arc shield of the present invention;

FIG. 2 is an enlarged sectional view of the embodiment of FIG. 1 taken along the line II-II of FIG. 1;

FIG. 3 is a sectional view of a metal vapor lamp employing an alternative embodiment for the arc shield of the present invention;

FIG. 4 is a sectional view taken along the line IV-IV of FIG. 3;

FIG. 5 is a metal vapor arc lamp employing still another embodiment for the arc shield of the present invention; and

FIG. 6 is an enlarged sectional view taken along the line VIVI of FIG. 5.

Referring now in detail to the drawing, wherein like reference characters represent like parts throughout the several views, there is shown in FIGS. 1 and 2 a metallic vapor lamp generally designated 10. The lamp 10 generally comprises an envelope 12 of sintered translucent polycrystalline alumina which defines the arc tube. The are tube 12 is usually constructed of a ceramic alumina having an extremely high alumina content of 99.5% A1 0 which, although translucent rather than clear like quartz or glass, evidences exceedingly high light transmissivity. The arc tube is sealed off at each end by a metallic or ceramic disc or end cap 14 and 16. For purposes of illustration, end caps 14 and 16 have been shown to be in the drawings as metallic and may be of a refractory metal such as niobium or tantalum. A backup ring is generally empolyed in constructing the seal and is usually of the same material as the arc tube as for example, polycrystalline alumina backup rings 18 and 20. The end caps or discs and their associated backup plates are usually sealed together and sealed to the alumina arc tube in a vacuum furnace or reducing inert atmosphere while in the presence of a suitable sealing composition. Compositions for sealing the end disc or cap to a polycrystalline alumina arc tube are disclosed in application Ser. No. 529,792, filed Feb. 24, 1966, in Horst Lange for an Alkali Metal Vapor Lamp and by application Serial No. 562,016, filed June 30, 1966 by Richard B. Grekila, Shih-Ming Ho, William J. Knochel and Francis C. M. Lin for Sealing Compositions for High Temperature Metal Vapor Lamps.

In the usual construction of these lam-ps, at one end a strip metallic lead-in conductor 22 is spot welded or brazed to the end disc 14 with a conductive electrode support member 24 also welded or brazed to the other side of the end disc which carries an electrode 26. At the other end, a tubular lead-in conductor 28 extends through the disc or end cap 16 and carries, at its innermost end, an electrode support member 30 spot welded or brazed thereto which carries an electrode 32. The tubular lead-in conductor is sealed through the disc 16, as for example by brazing, to provide for the evacuation of the interior of the alumina arc tube envelope as well as to provide for the insertion into the arc tube of the discharge sustaining media.

In accordance with the present invention, metallic arc shields or deflectors 34 and 36 are respectively interposed between electrode 26 and end disc 14 and electrode 32 and end disc 16. The arc shields or deflectors 34 and 36, as shown in FIG. 1, are disc shaped and of a diameter slightly less than the inside diameter of the arc tube. The discs 34 and 36 are mounted on the electrode stem but could equally well be mounted on the electrode support members 24 and 30. When a lamp constructed in accordance with this invention is started, should the are not strike between the electrodes during warm-up, it has been found that the arc will strike to the arc shield and not to the end disc or end disc sealing areas and thus destruction of the sealed lamp is prevented. It has additionally been found that the shield also serves to dissipate and reflect the heat generated at the electrode during operation thus preventing severe localized overheating of the end disc or end disc seal and hence is effective in reducing temperatures near the ends of the arc tube in those instances where extremely high temperatures are not critical to the substantial ionization of the alkali metal, alkaline-earth metal or metal vapor employed in the lamp.

While the disc type tantalum or molybdenum shield is quite effective in diverting the are from the ceramic to metal seal area and to the end disc during starting, in some instances, the reflective characteristics described above, render the end temperatures of the arc tube such that maximum light outputs cannot be attained due to lower metal vapor pressures. In order to substantially minimize this reflective effect, radially extending curved fins as shown in FIGS. 3 and 4 can be substituted for the metallic discs. These radially extending curved fins 44 and 46 while not only protecting the ceramic to metal seal areas as well as the end discs from the striking back of the are further permit heat generated at the electrodes to not only be radiated to the ends of the arc tube but also conducted to the metal end discs.

For example, in an arc tube of 3% inches length and of an inch outside diameter, a tantalum arc shield with radially extending curved fins of the type shown in FIGS. 3 and 4 constructed of a tantalum band 0.0005 inches thick and between to inch wide was employed in place of the disc type are shield shown in FIGS. 1 and 2. The lamp was charged with a sodium, mercury and argon discharge sustaining filling and produced a luminous efliciency of more than 100 lumens per Watt where previously only 70 lumens per watt were obtainable. It was further observed that the arc voltage across the electrodes increased from 30 volts to from 55 to 60 volts.

Another feature for increasing the efliciency of a high temperature gas discharge lamp is shown in FIG. 3. When the lamp is exhausted and filed with its discharge sustaining filling, tubulation 28 must be sealed off. Generally this tipping off is accomplished by squeezing and resistance spot welding the tubulation which then serves only as a lead-in conductor for the electrode 32. Efficiency of the lamps is sometimes reduced by the condensation of the metal vapors such as sodium and mercury in the cavity which remains in the tubulation which is in communication with the interior of the lamp after tip-off. This of course occurs because of the lower temperatures within the tubulation cavity, particularly in that area which extends exteriorly of the lamp through the end disc 16. It has been found that if a refractory metal plug 50, for example a plug of tungsten, molybdenum, tantalum, stainless-steel or nickel plated iron, is placed within the tubulation prior to tip-off that the problems of condensation during operation of the lamp are eliminated and elficiency of the lamp accordingly inQIFflWd- When lamps are constructed of A inch outside diameter and smaller polycrystalline alumina tubing, some diificulty is encountered in designing and constructing the radially extending curved fins of the FIGS. 3 and 4 embodiment small enough to fit within the tubing. As shown in the FIGS. 5 and 6 embodiment, Where extremely narrow polycrystalline alumina tubing is used, effective shielding of the ceramic to metal seal area and the end disc or cap can be accomplished by empolying an arc shield which consists of a plurality of radially extending spoke-like struts. The spoke-like strut configuration shown at '54 and 56 in the FIGS. 5 and 6 embodiment have been found to prevent the striking back of the arc during warm-up of the lamp and also permits the retention of high temperatures in the area of the lamp between the electrode and its associated end disc or cap. As in the FIG. 3 embodiment, an effective shield of radially extending spoke-like struts constructed of 0.005 inch thick by to inch wide tantalum or molybdenum bands have produced the desired results.

As can be seen from the foregoing improved metal vapor lamps constructed in accordance with the present invention effectively prevent the destruction of the ceramic or metal end disc or cap and the metal to ceramic seal areas during the starting of the lamp by effectively shielding those areas from the striking of an arc between the electrode and those areas while the lamp is warming to a sufficient temperature to sustain an are between the spaced electrodes.

Since numerous changes may be made in the above described apparatus, and different embodiments of the invention may be made without departing from the spirit thereof, it is intended that all the matter contained in the foregoing description or shown in the accompanying drawings, shall be interpreted as illustrative and not in a limiting sense.

We claim as our invention:

1. An electric discharge lamp comprising:

(a) a hollow elongated light transmissive ceramic envelope;

(b) first and second refractory closure members sealingly engaging the ends of said envelope;

(c) first and second lead-in conductors electrically connected through said first and second closure members respectively and extending both internally and externally of said envelope;

((1) first and second electrodes connected to and operatively supported within said envelope by said first and second lead-in conductors respectively; and

(e) refractory metal are intercepting means including a plurality of radially extending spoke-like struts electrically connected to and supported on each of said lead-in conductors intermediate each of said electrodes and its adjacent closure member, said spoke-like struts of each of said arc intercepting means projecting transversely of the axis of said envelope whereby any are striking past an electrode toward said closure member will be intercepted by said arc intercepting means.

2. A gas discharge lamp according to claim 1 wherein at least one of said lead-in conductors is tubular and filled with a metallic plug.

3. A gas discharge lamp according to claim 2 wherein said metallic plug is a refractory metal.

4. In an electric discharge lamp having an elongated hollow light transmissive ceramic envelope, closure members sealing off the open ends of said envelope, and lead-in conductors electrically connected through said closure members and supporting a pair of opposed electrodes within said envelope, the improvement comprising:

(a) refractory metal are interceptor means mounted within said envelope intermediate said electrodes and their adjacent closure members and having a plurality of curved fins extending transversely of said envelope a distance only slightly less than the internal diameter of said envelope to thereby prevent the striking of an are between an electrode and the opposite closure member during warm-up of said lamp.

5. A gas discharge lamp according to claim 4 wherein at least one of said lead-in conductors is tubular and is filled with a metallic plug.

6. A gas discharge lamp according to claim 5 wherein said metallic plug is a refractory metal.

7. An electric discharge lamp comprising:

(a) a hollow elongated light transmissive ceramic envelope;

(b) first and second refractory closure members sealingly engaging the ends of said envelope;

(c) first and second lead-in conductors electrically connected through said first and second closure members respectively and extending both internally and externally of said envelope;

(d) first and second electrodes connected to and operatively supported within said envelope by said first and second lead-in conductors respectively, at least one of said lead-in conductors being tubular and filled with a metallic plug to thereby reduce the enclosed volume behind said electrode; and

(e) refractory metal are intercepting means electrically connected to and supported on each of said lead-in conductors intermediate each of said electrodes and its adjacent closure member, a portion of each of said are intercepting means projecting transversely of the axis of said envelope whereby any are striking past an electorde toward said closure member will be intercepted by said are intercepting means.

8. A gas discharge lamp according to claim 7 wherein said metallic plug is a refractory metal.

References Cited UNITED STATES PATENTS 2,060,043 11/1936' COX 313-33 2,174,682 10/ 1939 Beggs 220-22 X 2,454,572 11/1948 Roovers 220-22 X 2,971,110 2/1961 Schmidt 313-227 X 2,087,735 7/1937 Pirani et al 313-42 JAMES W. LAWRENCE, Primary Examiner. R. L. JUDD, Assistant Examiner.

US. Cl. XlR. 

